It is well known to mount the end of a seat belt on a vehicle body via a linear belt retracting mechanism including a carriage movable on a track. The carriage is typically moved in a belt retracting direction by a cable connecting the carriage with a spring biased reel for winding the cable. A lock is provided to selectively prevent belt extension and is preferably a pawl or the like mounted on the carriage and selectively actuatable to engage the track. The lock is operated by a sensing means responsive to a condition of rapid belt unwinding or a condition of predetermined vehicle acceleration. The sensing means is preferably located at a fixed point on the vehicle body instead of on the moving carriage.